Writable optical recording medium

ABSTRACT

The present invention relates to a writable optical recording medium ( 500 ), and in particular to a write-once optical record carrier comprising a substrate ( 520 ) carrying a recording stack ( 510 ) which recording stack comprises a recording layer ( 516 ), made of PEDOT and/or derivatives thereof.

The present invention relates to a writable optical recording mediumcomprising a substrate carrying a recording stack with at least arecording layer. In particular, it relates to a write-once opticalrecord carrier.

Writable optical recording media have seen an evolutionary increase indata capacity by increasing the numerical aperture of the objective lensand a reduction of the laser wavelength. The total data capacity wasincreased from 650 MB (CD, NA=0.45, λ=780 mn) to 4.7 GB (DVD, NA=0.60,λ=650 nm) to finally 25 GB (Blu-ray Disc (BD), NA=0.85, λ=405 nm).Whereby, throughout all media two different writing principals areapplied: dye recording in case of write once CD-R and DVD+R andphase-change recording in case of rewritable CD-RW, DVD-RAM, DVD-RW,DVD+RW, and BD-RE.

Dye recording type media, in particular, write once optical discs aretypically composed of a polycarbonate substrate having a recording layermaterial applied on a first surface thereof. The recording layermaterial is a composition made of a photochromic compound—hereinreferred to as dye—and a (conjugated) polymer component. The compositionis optimized, in particular, the dye is chosen in order to fulfillconditions such as thermal stability, durability, writing sensitivitydepending on the applied laser frequency and optical contrast of areflected reading beam between written and unwritten portions of therecording medium. Known dye materials are cyanine, phthalocyanine andmetallized azo.

A writing laser beam entering an optical record carrier is focused onthe recording layer described above and is partially absorbed by therecording layer material. Thereby, the recording layer material isheated and decomposed, i.e. it durably and irreversibly changes itsstructure. More precisely, in the above composition electronicinteraction and/or steric arrangement between the dye and the conjugatedpolymer varies when it is heated and, moreover, interaction amongmolecules of the dye changes so that the state of aggregation of the dyemolecules change. These changes can be observed as a change in opticalabsorption and/or reflection behavior of the recording layer. Also, somemechanical deformation of the recording stack may occur.

A reading beam striking a mark written in the manner described abovewill be partially scattered by the decomposed area. Consequently, theintensity of the light reflected at said reflective metal layer dependson whether the reading beam strikes a mark or passes the recording layeralmost undisturbed (unwritten areas).

The growing demand for higher data capacity and the general demand forlower cost has motivated manufacturers to seek for new recordingmaterials. In U.S. Pat. No. 5,648,135 for example a recording layer isproposed for a recordable optical disc in the wavelength range of 780 nmand 830 nm comprising a conjugated polymer and a dye which dye isselected from the group of phthalocyanine dyes,tetrapyradinoporphyradine dyes, naphthalocyanine dyes and nickel dithiolcomplexes.

Similarly, in U.S. Pat. No. 5,443,940 an optical recording medium isproposed that contains a photochromic compound and a polymer component.

WO 97/08692 addresses the problem that some dyes do not have high enoughdichroism (absorption of light being polarized parallelly to theorientation of the dye molecules divided by absorption of light beingpolarized perpendicularly) and other dyes were found not to be absorbentin the desired wavelength range. Effort was spent on the search for asuitable dye material providing a high dichroism and the ability toalign with the orientation of a liquid crystalline material.

Although, the blu-ray disc has been introduced as a re-writable opticaldata storage system based on the phase-change writing principalmanufacturers currently also work on the development of recordingmaterials, in particular dyes, that can be used at the BD-wavelength of405 nm. Also the availability of recording materials, dyes or organicsubstances, that show excellent recording characteristics, in particularat high speed recording conditions, is a preference for futuregeneration optical discs. Furthermore, inorganic recording layers areconsidered as candidate for write-once applications. However, theproduction of that kind of materials often is very elaborate.

An objective of the present invention therefore is to provide a writableoptical recording medium employing a competitive recording materialwhich is easily producible and applicable.

According to a first aspect of the present invention this object isachieved by a writable optical recording medium as described in theopening paragraph which is characterized in that said recording layer ismade of PEDOT and/or PEDOT-derivatives.

Note that the term PEDOT consecutively will be used to denote both thePEDOT-polymer and PEDOT-derivatives. Several polymerization processeshave been described to obtain PEDOT (also referred to as PEDT) andPEDOT-derivatives in a stable doped form, see Groenendaal, L., Jonas,F., Freitag, D., Pielartzik, H., Reynolds, J. R., in“Poly(3,4-ethylenedioxy-thiophene) and its derivatives: past, present,and future”, Adv. Mater., 12, 481 2000. For example, Bayer currentlymarkets a Baytron® P grade, based on an oxidative polymerization processyielding a PEDOT/polystyrene sulfonic acid blend that can be processedas a dispersion in water. Though PEDOT is known (see also EP-A 339 400and WO 01/90212) its usability as optical recording layer material hasnot been recognized so far.

PEDOT displays the following properties: good resistance to hydrolysis,good photo- and thermal and electrochemical stability. The highdecomposition temperature of PEDOT of 300° C. to 400° C. is about50-100° C. higher than that of typical recording dyes. Therefore, PEDOTis expected to have a higher stability with respect to archival andshelf life than other known (organic) recording layer materials.

Due to its properties PEDOT is very easily processible. The polymer isdispersible in water. Coatings of PEDOT therefore can be applied byconventional coating methods such as brushing, printing, ink-jetprinting, off-set printing, spraying, roller-coating, and, preferably,spin-coating. Spin-coating is possible on a huge variety of substratesincluding glass, silicon, chromium, and gold. Furthermore, virtually allplastics can be coated with PEDOT, including polycarbonate,polyethylene, polyethylene terephthalate, polyamide, and polypropylene.

Whereas, all known recording methods and write-once recording materialspresently applied for CD-R, CD-RW, DVD+R, DVD-RAM, DVD-RW, DVD+RW, andproposed for BD make use of an additional dye material in order toimprove the absorption of the recording layer the absorption of PEDOTitself is perfectly adopted for the short wavelength as applied byBlu-ray Disc technology. Thus, PEDOT can be considered as a superior andcheap replacement of current recording materials containing dyes.

According to a second aspect which constitutes a further development ofthe first aspect of the invention the recording stack further comprisesa first and second reflector layer arranged adjacent on opposite sidesof the recording layer.

According to a third aspect which constitutes a further development ofthe second aspect of the invention the first and second reflector layerscomprise silver.

According to a fourth aspect which constitutes a further development ofone of the first to third aspects of the invention the recording stackfurther comprises a first and second dielectric layer arranged adjacenton opposite sides of the recording layer.

According to a fifth aspect which constitutes a further development ofthe fourth aspect of the invention the first and second dielectriclayers comprise ZnS—SiO₂.

According to a sixth aspect which constitutes a further development ofthe fifth aspect of the invention the recording medium comprises therecording stack according to the third aspect of the invention formed onsaid substrate as a second recording stack, a spacer layer formed onsaid first recording stack opposite the substrate, the recording layeraccording to the fifth aspect of the invention formed on said spacerlayer as a first recording stack, and a cover layer formed on said firstrecording stack opposite the spacer layer.

According to a seventh aspect which constitutes a further development ofone of the first to sixth aspects of the invention the recording mediumfulfils the requirements of a Blu-ray Disc.

According to an eighth aspect of the present invention the aboveobjective is further achieved by the use of PEDOT and/orPEDOT-derivatives as recording layer material in a writable opticalrecording medium.

The above an other objectives, features and advantages of the presentinvention will become apparent from the following description ofpreferred embodiments thereof taken in conjunction with the accompanyingdrawings in which

FIG. 1 illustrates the chemical structure of PEDOT;

FIG. 2 illustrates the chemical structure of Baytron® P PEDOT/PSS blend;

FIG. 3 shows the measured index of refraction parallel and perpendicularto the surface plane of thin PEDOT films;

FIG. 4 shows ellipsometry measurements of the index of refraction (n)and absorption coefficient (k) of a thin PEDOT film;

FIG. 5 illustrates a cross sectional view of a writable opticalrecording medium according to a preferred embodiment of the presentinvention;

FIG. 6 shows the optical performance as a function of the thickness of afirst silver layer in the writable optical recording medium according toFIG. 5;

FIG. 7 illustrates a cross sectional view of a writable opticalrecording medium according to another preferred embodiment of thepresent invention;

FIG. 8 shows the optical performance as a function of the thickness of afirst silver layer in the writable optical recording medium according toFIG. 7;

FIG. 9 illustrates a cross sectional view of a dual-layer writableoptical recording medium according to a third preferred embodiment ofthe present invention;

FIG. 10 shows the optical performance of the dual-layer writable opticalrecording medium according to FIG. 9.

The chemical structure of PEDOT (poly(3,4-ethylenedioxythiophene) asillustrated in FIG. 1 has been published by Petterson et al. in 1999. Ascan be seen in FIG. 1, PEDOT is a conducting polymer based on aheterocyclic thiphene ring bridged by an diether. This means, it has thesame conjugated backbone as polythiophene for example. As mentionedearlier above, however, in accordance with the present invention theterm PEDOT herein is not to be understood as denotation for the basicpolymer as shown in FIG. 1. PEDOT is rather to be understood ascomprising both the polymer and its derivatives. An example of aPEDOT-derivative is given in FIG. 2, namely, a Baytron® P PEDOT/PSSblend.

The dependency on the wavelength of an incident light beam of opticalproperties of a PEDOT film deposited on glass with the optical axisparallel to the sample normal have been measured by Petterson et al.(1999). Measured values of the ordinary index of refraction (N₀ orn_(//)) parallel to a sample surface of a thin PEDOT film (upper solidline) and the extraordinary index of refraction (N_(e) or n_(⊥))perpendicular to the sample surface of that film, but aligned with theoptical axis (upper dotted line) are shown in FIG. 2. Furthermore, theabsorption coefficient (k_(//)) parallel to the sample surface of thethin PEDOT film (lower solid line) and the absorption coefficient(k_(⊥)) perpendicular to the sample surface (lower dotted line) areshown in this Fig. As can be seen from the measurements the index ofrefraction and absorption coefficient of PEDOT are anisotropic.

Further, we have measured the index of refraction n and absorptioncoefficient k of a 130 nm thick PEDOT layer (according to the formulashown in FIG. 1) spin-coated on a glass substrate. The opticalproperties n and k were measured with an ellipsometer at angles between50-90°. The results are given in FIG. 4. Therein three sets of (n,k)-values are given for three different samples (init #1 to #3). Themeasured n and k can be denoted as effective parameters since noanisotropy is taken into account. Still, the values agree very well withthe ordinary index of refraction (N₀ or n_(//)) as measured by Pettersonet al. (1999), compare FIG. 3.

We have further measured the optical properties of the decomposed PEDOTlayer. Decomposition was achieved by heating the sample up to 400° C. Adetectable difference in the index of refraction is observed at 405 nmwavelength between the initial (n(init)=1.45) and decomposed state(n(decomposed)=1.65). The measurements reveal that the absorptioncoefficient hardly changes upon decomposition (k=−0.1). Therefore, PEDOTcan serve as recording medium in a write-once BD-R disc. Themeasurements further illustrate that PEDOT can be used at otherrecording wavelengths as well, such as the 780 nm wavelength as used inCD optical devices and 650 nm as used in DVD optical devices.

As can be seen in FIG. 5, a writable optical recording medium 500 inaccordance with a first embodiment of the present invention comprises arecording stack 510. The recording stack 510 comprises, top-down—withregard to the direction of the incident light beam as indicated by arrow502—a first silver reflector or mirror layer 514, also referred to asM₁, a recording layer 516 made of organic PEDOT according to the formulashown in FIG. 1, also referred to as O, having a thickness of 60 nm, andadjacent to said recording layer 516 opposite the first reflector layer514 a second reflector or mirror layer 518 also referred to as M₂ whichin this embodiment is about 20 nm thick. The mirror layers 514, 518 inthis example are made of silver but may as well consist of other metalor metal alloy, preferably aluminium, a mixture of Al with somepercentage of Ti, or gold for example. The recording layer may furtherconsist of any PEDOT-derivative. The bottom layer of the recordingmedium 500 underneath the recording stack 510 is a substrate 520preferably made of polycarbonate. The above arrangement of layers,namely the first reflector layer 514, the recording layer 516, thesecond reflector layer 518 is laminated onto the substrate 520 inreverse order. A cover layer 512, herein also referred to as C islaminated on top of the recording stack for protection. This cover layermay be made of e.g. Sylgard 184 or of polycarbonate and in case of aBD-disc, this layer may be about 100 μm thick. The record carrier may aswell be of the air-incidence type. The embodiment according to FIG. 5represents a write-once recording stack.

A 405 nm blu-ray laser beam, as indicated by the arrow 502, is generatedby a writing unit positioned at a predetermined position with respect tothe record carrier. It enters the stack from the side of the M₁ layer514 and is focused by said writing unit on the recording stack 510 and,more precisely, on the recording layer 516. In case a cover layer 512 ispresent the light is focused through that cover layer. Then, the energytransported by the electromagnetic radiation is partially absorbed inthe recording stack 510 and, in particular, in the PEDOT material of therecording layer 516. The absorbed portion of the light inducessufficient heat for locally changing the optical properties of the PEDOTmaterial (decomposition) in a manner as described above.

FIG. 6 illustrates these optical properties and, more precisely, thereflection of the initial and decomposed PEDOT stack and correspondingcontrast as a function of the thickness of the first silver layer M₁ inthe cover-M₁-O-M₂-substrate-stack as described in conjunction with FIG.5. The reflection of the initial state is indicated by a dashed line,the reflection of the decomposed state is represented by a dotted line,and the corresponding contrast—which can be denoted as normalizedreflection difference: Contrast=(R(init)−R(decomposed))/R(initial)—isindicated by a full line. It was found that an M₁-layer thickness of 12nm results in the best optical properties: a high reflection difference,an initial reflection of 10% and a contrast of 95%.

According to another embodiment of the present invention as shown inFIG. 7, a writable optical recording medium 700 comprises a recordingstack 710 wherein an organic PEDOT recording layer 716, also referred toas O, is sandwiched between two dielectric layers 714, 718. The upperdielectric layer 714 (with regard to the direction of the incident lightbeam as indicated by arrow 702) is herein also referred to as I₁ orfirst dielectric layer, the lower one 718 is referred to as I₂ or seconddielectric layer. The PEDOT recording material comprises the chemicalstructure according to the formula shown in FIG. 1. According to thisembodiment it has a thickness of 100 nm. It may also consist of anyPEDOT-derivative such as that shown in FIG. 2. The lower dielectriclayer 718 according to this embodiment is 43 nm thick. Both dielectriclayers 714, 718 are made of ZnS—SiO₂, e.g. (ZnS)₈₀(SiO₂)₂₀. Therecording I₁—O—I₂ stack 710 may be employed as semitransparent stack ina dual layer optical recording disc. The bottom layer 720 of therecording medium 700, therefore, may be a spacer layer on top of a deep(with respect to the incident light beam 702) recording stack in a dualrecording layer disc (see FIG. 9) or a (dummy-)substrate preferably madeof polycarbonate for example. The above arrangement of layers, namelythe first dielectric layer 714, the recording layer 716, the seconddielectric layer 718 is laminated onto the bottom layer 720 in reverseorder. A cover layer 712, herein also referred to as C, typically madeof Sylgard 184 or of polycarbonate, is laminated on top of the recordingstack for protection. Other (semi-) transparent materials such as Si₃N₄,SiC, Al₂O₃, etc may be used as well as dielectric layer in the claimedrecording stacks based on PEDOT or/and PEDOT derivatives recordinglayers.

Again, a 405 nm blu-ray laser beam, as indicated by the arrow 702,generated by a writing unit enters the stack from the side of the I₁layer 714 and is focused by said writing unit on the recording stack 710and, more precisely, on the recording layer 716. Then, the energypartially absorbed in the recording stack 710 and, in particular, in thePEDOT material of the recording layer 716 and/or in the dielectricZnS—SiO₂ layers I₁ 714 and I₂ 718 at sufficiently high laser powersinduces a temperature rise in the PEDOT material that exceeds thedecomposition temperature of the PEDOT material. In this way marks arewritten in the recording layer material.

The optical performance of the cover-I₁—O—I₂-spacer stack according toFIG. 7 as a function of the thickness of the first ZnS—SiO2 dielectriclayer I₁ 714 is shown in FIG. 8. The optical performance of the stack:initial reflection (dotted line), contrast (full line) and effectivetransmission (dashed line; whereby, effective transmission=¾transmission of the initial state and ¼ transmission of the decomposedstate) is found to be best at a I₁ layer thickness of 66 nm. Then theinitial reflection being about 10% of the incident light, the contrastbeing 89% and the effective transmission after recording of 57% makesthis stack suitable for dual-layer (or even multi-layer) recording.Therefore, the embodiment according to FIG. 7 represents a write-oncerecording stack and may be employed as a semitransparent top stack (withregard to the incident light beam), also referred to as L₀-stack, in adual-layer disc.

Such a dual layer arrangement is shown in FIG. 9. A writable opticalrecord carrier 900 in accordance with a third embodiment of the presentinvention comprises a lower L₁-recording stack 911 (with regard to thedirection of the incident light beam as indicated by arrow 902)deposited onto a substrate 928. On top of the L₁ stack a firstsemi-transparent recording stack L₀ 910 may be deposited with a spacerlayer 920 in between both stacks 910 and 911. Thereby, a I₁—O—I₂recording layer as described in conjunction with FIG. 7 is proposed asfirst recording stack L₀. As outlined above, this stack has an effectivetransmission of 57%. A cover layer 912 may be arranged on top of thedual layer arrangement. A PEDOT layer sandwiched between two silverreflector layers is proposed as second recording stack L₁. The layout ofthe writable optical disc then becomes (bottom-up): substrate 928-M₂926-O 924-M₁ 922-spacer 920-I₂ 918-O 916-I₁ 914-cover 912. The I₁ layeris 66 nm thick, the PEDOT layer in the upper recording stack L₀ is 100nm thick, and the I₂ layer is 43 nm thick. The thickness of the PEDOTlayer in the lower M₁-O-M₂ stack is 60 nm, the deep silver layer M₂ is100 nm thick.

FIG. 10 shows the optical properties of the lower recording stack L₁ inthe writable optical record carrier according to FIG. 9. In particular,the reflection (dashed line) and the effective reflection (dotted line;after having passed twice the first recording stack) of the initialstate and the corresponding contrast (full line) are shown as a functionof the thickness of the first silver layer M₁. It can be seen that athickness of 22 nm gives an effective reflection of 10.5%, and acontrast of 83%.

It is noted that the present invention is not restricted to the abovepreferred embodiments. As would be obvious to one skilled in the art,many variations and modifications of the invention may be effectedwithout departing from the spirit and the scope of the novel concepts ofthe disclosure. In particular, the application of a PEDOT recordinglayer is neither restricted to Blu-ray Disc technology nor to write onceapplications. Several arrangements of layers are possible in addition tothose discussed above, e.g. C—I₁—O—I₂-M-S. A dual layer arrangement isalso possible having a combination of different recording layers,whereby only one recording layer is made of PEDOT or a PEDOT-derivativeand the other may comprise a recording dye or phase-change material, forexample. Furthermore, a multi layer arrangement is possible having morethan two recording layers, whereby at least one of these recordinglayers is made of PEDOT or a PEDOT-derivative. The dielectric layermaterial may be composed differently from the above embodiments. Furtherthe reflector layers can be made of a different metal or metal alloy.The cover layer and the substrate materials are not restricted to thematerials mentioned above. The thickness of all of the mentioned layersmay be chosen differently from the ranges given in accordance with theabove embodiments.

It should further be noted that an optical recording medium according tothe present invention can also be used for multi level recording,whereby an increased number of data bits is stored in the same recordcarrier by applying multiple reflection levels, namely more than tworeflection levels as known from conventional optical discs describedabove. These different reflection levels can be obtained by pit-width orpit-depth modulation, for example created by a higher laser power.

1. A writable optical recording medium (500, 700, 900) comprising asubstrate (520, 720, 920) carrying a recording stack which recordingstack (510, 710, 910, 911) comprises a recording layer (516, 716, 916,926), characterized in that said recording layer is made of PEDOT and/orPEDOT-derivatives.
 2. A writable optical recording medium according toclaim 1, characterized in that the recording stack further comprises afirst (514, 922) and second reflector layer (518, 928) arranged adjacenton opposite sides of the recording layer.
 3. A writable opticalrecording medium according to claim 2, characterized in that the firstand second reflector layers comprise silver.
 4. A writable opticalrecording medium according to claim 1, characterized in that therecording stack further comprises a first (714, 914) and second (718,918) dielectric layer arranged adjacent on opposite sides of therecording layer.
 5. A writable optical recording medium according toclaim 4, characterized in that the first and second dielectric layerscomprise ZnS—SiO₂.
 6. A writable optical recording medium (900)according to claim 3, which recording medium comprises, the recordingstack according to claim 3 formed on said substrate as a secondrecording stack, a spacer layer formed on said first recording stackopposite the substrate, the recording layer according to claim 5 formedon said spacer layer as a first recording stack, and a cover layerformed on said first recording stack opposite the spacer layer.
 7. Awritable optical recording medium according to claim 1, characterized inthat it fulfils requirements of a Blu-ray Disc.
 8. Use of PEDOT and/orPEDOT-derivatives as recording layer material in a writable opticalrecording medium.